1. Field of the Invention
The present invention relates to a control apparatus for a continuous hot rolling mill for controlling a thickness of a rolled material on each stand of a tandem rolling mill, an interstand tension and a height of each of loopers disposed between the respective stands.
2. Related Background Art
The thickness of a strip is defined as a part of the criteria for evaluating the final product in both hot rolling and cold rolling. This thickness is one of the most essential properties in the product. Previous methods of thickness control have included gauge meter AGC (Automatic Gauge Control), MMC (Mill Modulus Control) and X-ray monitor AGC.
Particularly, the rolled material in the hot rolling is weak in terms of resistance to deformation at high temperature. If the tension thereof is large, the rolled material is easily ruptured. Because of this, a hot rolling mill includes loopers. The tension is controlled by the looper, and the looper height is controlled in terms of enhancing a transferability of the material.
When a roll gap is controlled for improving accuracy of thickness over the rolled material, the interstand tension or the looper height fluctuates. Further, there exists such relationships that the fluctuation in the tension leads to a fluctuation in the thickness; and if the looper height fluctuates, the tension fluctuates as well as causing a fluctuation in the thickness.
According to the thickness control in the prior art, the tension of the rolled material and the looper height have been controlled by the PI control without restraining an interference between the tension and the looper height.
On the other hand, Japanese Patent Laid-Open Publication No. 2-211906 discloses a control method which involves an application of so-called LQ (Linear Quadratic) control for determining control gains by an evaluation function in the quadratic form to control the thickness, the interstand tension and the looper height in combination.
As explained earlier, according to the thickness control such as the gauge meter AGC, etc., the roll gap is independently controlled without employing a value of tension of the rolled material which influences the thickness. Consequently, a manipulated variable becomes excessive enough to induce an interference. This may result in a response concomitant with a large overshoot. Further, the thickness and roll gap values are also not used in the tension control. A speed change quantity of a rolling mill driving main motor is additionally calculated as a manipulated variable of the tension control. The response, however still tends to contain a large overshoot.
Further, the method based on the LQ control theory poses difficulty in determining a causality between a weight matrix Q and R in the following evaluation function J and an actual process response. A general practice is to determine the control gains by seeking Q and R in the manner of trial-and-error which realizes a proper response of the whole control system. ##EQU1## where y is the output or the state quantity of the controlled process, W is the manipulated variable given to the controlled process by the controller, y.sup.T is the transposition of y, and W.sup.T is the transposition of W.
The trial-and-error action is repeatedly performed in the LQ control. Hence, a design of the control system and an adjustment of the plant are very time-consuming. According particularly to the technique disclosed in Japanese Patent Laid-Open Publication No. 2-211906, an interstand transfer lag is approximated by a first-order lag; and the thickness, the tension and the looper height are conceived as state quantities. It is therefore considered that a very high-order state equation be prepared for expressing the controlled process. If the order of the state equation is high, Q and R are hard to adjust.
In addition, the interstand transfer lag should be originally expressed as a dead time element. The transfer lag is, however, approximated by the first-order lag in this technique. Accordingly, a deterioration in terms of an accuracy of the model is also considered. Moreover, according to the method based on the LQ control theory, it is required that an analytically unsolvable Riccati's (differential) equation be solved numerically. There also exists such an inconvenience that a general equation for the optimum control gains containing variables can not be obtained.
Note that a general practice according to a method which does not obtain the general equation but utilizes a gain table is to previously prepare the gain table by seeking the control gains adjusted to properties of the rolled material and rolling conditions and refer to this table when using the control gains. It therefore follows that a determination, a retention and a management of values in the gain table are very time-consuming.
Further, describing all cases in the gain table is almost impossible. There is no alternative but to approximate the gains from a table similar to rolling conditions which do not exist in the gain table, and, therefore, a decline in control performance may be expected.